The present invention relates to a stencil printer and, more particularly, to a stencil printer capable of producing printings with a plurality of drums.
A digital thermal printer using a stencil is extensively used for its simple configuration and easy operation. The printer includes a thermal head carrying an array of fine heating elements thereon. While the thermal head is held in contact with a thermosensitive stencil being conveyed, the heating elements are selectively energized by pulses in accordance with image data in order to perforate, or cut, the stencil by heat. After the perforated stencil, i.e., master has been wrapped around a hollow cylindrical porous drum, ink is transferred from the drum to a sheet via the perforation pattern of the master so as to print an image on the sheet. Specifically, an ink roller is disposed in the drum while a press roller is located face the ink roller in the vicinity of the drum. When the press roller is pressed against the drum, the ink is caused to ooze out from the inner periphery of the drum to the outer periphery of the same via the master. As a result, the ink is transferred from the drum to the sheet.
The above printer is capable of producing a desired number of printings, as follows. A master derived from a document of first color is wrapped around the drum, and an ink image of first color is repeatedly transferred to a desired number of sheets via the master. After a master derived from a document of second color has been wrapped around the drum, the sheets carrying images of first color are again fed from a sheet feed section to the drum one by one so as to transfer ink images of second color. This kind of procedure has the following problems left unsolved. Assume that after the transfer of ink images of first color to the desired number of sheets, but before the transfer of ink images of second color to the same sheets, the operator desires to increase the number of printings. Then, the operator must again set a desired number of sheets for the first color and repeat the printing operation all over again, resulting in time-and labor-consuming work. Moreover, because the images of second color are transferred to the sheets just after the transfer of the images of first color, the ink on the sheets deposit on and smear, e.g., the sheet feed section.
In light of the above, Japanese Patent Laid-Open Publication No. 7-17121, for example, proposes a color stencil printer including a plurality of drums arranged side by side in an intended direction of sheet transport at a preselected interval. A master derived from an image of particular color is wrapped around each of the drums. An intermediate transport device is arranged between the drums in order to transport a sheet carrying an image transferred from upstream one of the drums in the above direction to a downstream one of the drums. With this configuration, the printer is capable of effecting simultaneous multicolor printing in a single sheet feed procedure. The intermediate transport device transports a sheet at a constant speed while the drums each rotates at a constant speed synchronous with a sheet feed timing. In this condition, a sheet meets an ink image formed on each drum at a print position assigned to the drum.
However, the problem with the conventional stencil printer having the simultaneous multicolor printing capability is that ink transferred from the upstream drum to the sheet deposits on the master wrapped around the downstream drum and then deposits on the next sheet brought from the upstream drum. Let this occurrence be referred to as double printing. The amount of double printing is dependent on the print conveyance speed of the individual drum and the conveyance speed of the sheet. Further, in the case of stencil printing, the press roller presses the sheet against the associated drum in order to transfer an ink image from the drum to the sheet. As a result, the area of the ink image and therefore the amount of ink to deposit on a sheet varies in accordance with the size of the ink image and that of the sheet.
It follows that the time when the sheet adhered to the drum at the time of printing is peeled off from the drum varies in association with the amount of ink. This disturbs the position where the intermediate transport device starts conveying the sheet, and therefore the timing for feeding the sheet to the downstream drum. Consequently, the timing for transferring an ink image from the downstream drum to the sheet is deviated, resulting in the misregister between images and the previously stated double printing.
Another problem is brought about with the stencil printer including a plurality of drums when the sheet has a size or length greater than the distance between consecutive print positions respectively assigned to the upstream drum and downstream drum. Specifically, each drum is caused to rotate by a motor or similar drive source via a driveline including gears and a belt. It therefore sometimes occurs that the drums rotate at different peripheral speeds due to the deformation of belts and the production errors of gears. In this condition, it is likely that the sheet is slackened or pulled in the direction of sheet transport during printing. For example, assume that the peripheral speed of the downstream drum is higher than the peripheral speed of the upstream drum. So long as the length of the sheet is smaller than the distance between the print positions, the above difference in peripheral speed does not matter at all because the sheet is driven at the peripheral speed of the downstream drum as soon as its leading edge reaches the downstream print position and its trailing edge moves away from the upstream print position. However, if the length of the sheet is greater than the above distance, it bridges the upstream and downstream print positions and is pulled by the downstream roller in the direction of sheet transport. This is apt to dislocate the image printed on the sheet at the upstream print position or dislocates it relative to the image printed on the same sheet at the downstream print position, rendering the resulting color printing defective.
When the peripheral speed of the downstream drum is lower than the peripheral speed of the upstream drum, the sheet slackens on the intermediate transport device. The resulting color printing is also defective although the dislocation of the image printed on the sheet at the upstream print position or the dislocation thereof relative to the image printed on the same sheet at the downstream print position will be less noticeable than in the above-described case.
Technologies relating to the present invention are also taught in, e.g., Japanese Patent Laid-Open Publication Nos. 64-18682, 5-229243, 8-169628, 3-55276, and 1-290489.